Recreational vehicles (RVs) and pleasure boats generally contain two holding tanks, one for fresh water, and one for waste. The latter is generally called the waste tank, although the term "holding tank" is sometimes used also in the more limited sense to describe the waste tank. In the present application, the term "holding tank" refers to both the fresh water tank and the waste tank. The status of these tanks, i.e. whether full, partially full, or empty, must be monitored. While it is often acceptable to run out of potable water due to the water tank becoming empty, the situation with respect to the waste tank is completely different. The inability to utilize toilet facilities, especially in pleasure boats which may be a great distance from shore, creates a highly uncomfortable and medically undesirable constraint which is highly unsatisfactory. Moreover, although water is removed from the water tank by a pump, and therefore the water tank itself is not under pressure, the reverse is generally true with respect to holding tanks, where the commonly used toilets pump waste under pressure into the tank. Continuing to pump waste, either electrically, or manually, into a filled holding tank may result in rupture of the lines or the tank within the vehicle or boat, or may cause the tank to overflow through the vent, creating an extremely unsanitary condition.
Water tanks on recreational vehicles and pleasure boats are generally not monitored, due to the expense of providing a monitoring apparatus, and for other reasons which will become apparent in later discussion. Due both to the necessity of having a working sanitary system, as well as the potential problems which could be caused by leakage or overflow of waste from the waste tank, however, waste tanks are generally provided with a sensor in at least the higher cost vehicles and pleasure boats, especially those designed for long trips or, in the case of boats, where considerable use away from marinas or dockage is to be expected.
Level sensing means used in the past have not proven acceptable in the environments in which RVs and pleasure boats are used. For example, an analog level sensing device comprising a float attached to a cable, or a rheostat and the latter connected to a dial indicator, have been used in the past. Such devices are not acceptable for long-term use in either water tanks or waste tanks. In both applications, the constant violent sloshing of the tank contents during normal operation of vehicle, for example, when an RV encounters a large bump or depression in the road surface, or a pleasure boat encounters a wave, wake, or merely stormy weather, places great stress on the operating mechanisms of such devices, and thus their lifespan is greatly limited. It is not uncommon to replace such devices every one or two years of vehicle operation, due to bending or breakage of the components. In the case of waste tanks, the situation is exacerbated by the constituents of the waste itself. Human waste, as well as toilet paper and other material which occasionally may find its way into the waste tank, may foul the mechanism of such mechanical devices, rendering them inoperable even though component failure has not occurred. In either case, many owners prefer not to replace these components, therefore running a risk of an extremely discomforting or unsanitary condition, due to the cost of the replacement, and the cost and unsanitary nature of the replacement process.
To increase the reliability of such level sensors, hybrid electronic/mechanical sensors have been devised. In U.S. Pat. No. 4,140,996, for instance, is disclosed a device having a flexible PVC tubing inserted into the waste tank. The PVC tubing contains a magnetically actuated reed switch mounted at a predetermined level on the PVC tubing. A cylindrical float surrounds the PVC tubing, and contains a magnet which can activate the reed switch. As the float is proximate to the reed switch at only a given level of fill, an integrated circuit is necessary to monitor the past history of the fill indicators.
While such a device is an improvement over purely mechanical devices, it suffers from numerous drawbacks. First and foremost, free movement of the float up and down the PVC shaft is necessary in order for the device to function. In a water tank, where no solid material is available to impede the operation of the float, the device should work successfully provided that the fatigue encountered by the PVC tubing as it flexes due to the impact of sloshing water against the float does not fracture the PVC tubing over time. In a waste tank, the situation is different, however, as the nature of the material present may clog the space between the float and the tubing, thus creating erratic behavior or even total inactivation of the device. Second, the use of a complex integrated circuit monitoring device increases the cost of the unit dramatically, thus in a practical sense preventing its application on many low end recreational vehicles and pleasure boats, as well as encouraging non-replacement of failed components.
In U.S. Pat. No. 4,739,658, is disclosed a completely electronic monitoring device employing multiple conductivity probes, one for each level desired to be sensed. The conductivity probes and a reference probe are connected in a bridge circuit which is then monitored and converted to an appropriate visual display. The use of conductivity probes is problematic, however. In the case of water tanks, even the low amount of current flowing through the water in the tank may cause electrolysis or leaching of metallic ions from the conductivity probes into the water, thus decreasing the value of the water as a potable source. In the case of a waste tank, the conductivity probes may be coated with sludge, or may have solid material wedged between the conductive electrodes, thus again rendering the device erratic or inoperable. Finally, the metallic probes utilized in such systems are not generally acceptable for use in salt water environments, where corrosion is a severe problem. The necessity for having a complex bridge circuit and other electrical components increases the cost of the device.
A similar system is disclosed in U.S. Pat. No. 4,140,996. However, in the '996 patent, various conductivity probes in various tanks are multiplexed and utilized to provide a digital display indicating status of components. Such a system suffers from the same drawbacks as indicated for the '658 patent, and in addition requires even more complex electronic circuitry, thus further increasing the cost of the system.
In U.S. Pat. No. 3,600,946, is described an electronic level detector useful for detecting brake fluid level in an automotive or other vehicle braking system. The device disclosed in the '946 patent utilizes as the primary sensing element, a silicon semiconductive resistance material displaying both a positive and a negative temperature coefficient of resistance in series with a second device, preferably mounted to the wall of the tank, which monitors the ambient temperature, and which itself displays a positive temperature coefficient. The latter device may be made, for example, of nickel, platinum, copper, or iron, all of which have positive temperature coefficients of approximately 0.4% per degree Celsius.
In the '946 device, an incandescent indicator lamp is connected in series with the sensors and is energized by the vehicle power supply. Components are selected so as to keep the operating parameters of the device in the negative temperature coefficient region while the device is immersed in liquid, thus presenting a high resistance in the circuit, and preventing the lamp from energizing. If the liquid level should fall below the primary sensor, the resistance decreases, causing current to flow through the circuit and energizing the incandescent lamp. The necessity of employing two sensors for each level to be detected increases both the initial manufacturing costs as well as the cost of installation of the device. Moreover, an incandescent lamp which draws an appreciable amount of current, is utilized as a visual indicating element, and should the device be inadvertently left on while the vehicle is unattended, a current drain on the battery could render the vehicle inoperable after a relatively short period of time.
In U.S. Pat. No. 3,792,456 is disclosed a complex level detection device containing a positive temperature coefficient resistor mounted so as to be both thermally coupled to the liquid whose level is to be sensed, and electrically coupled through the liquid to the wall of the container in which the liquid is contained. A warning device is activated by the transistorized circuitry in the case where the temperature rise of the liquid exceeds a predetermined value, or the impedance of the device with respect to ground is altered as would be the case when the liquid level falls below the level of the sensing device. While the device of U.S. Pat. No. 3,792,456 is suitable for detection of low fluid level, the device is not suitable for detection of fluid levels higher than the sensing device. Moreover, the cost of the many electronic components necessary in the device increases the cost substantially, and the device is not suitable for use in polymer tanks, which are common on RVs and pleasure boats.
In U.S. Pat. No. 4,564,834 is disclosed a device suitable for detecting low levels of fluid in an engine crankcase. The device disclosed in the '834 patent includes two positive temperature coefficient elements, both located at the same height, but having different thermal loss coefficients. A voltage level detecting circuit monitors the difference in voltage drop across the two devices, and latches to light an appropriate indicator lamp when the oil level in the crankcase falls below the level of the detectors. The use of two positive temperature coefficient devices as well as the transistorized or integrated circuit voltage comparator increases the cost of the system undesirably. Moreover, the use of an incandescent lamp as a status indicator can cause an undesirable drain on the vehicle electrical system were the device to be operational when the alternator attached to the vehicle engine is not operating.
In U.S. Pat. No. 4,638,288 is disclosed a multiplexed monitoring device for various vehicle liquid systems employing PTC resistor sensors. The status of the PTC resistors are used to bias monitoring transistors which, in turn, activate a tone generator to supply an aural warning signal. The system employs many components, and thus the cost of the device is unacceptable for many applications. Moreover, the probes illustrated in the patent are susceptible to corrosion, thus limiting lifespan of the device in corrosive environments such as those encountered in salt water.
The unsuitability of the prior art devices for use in recreational vehicles, and especially pleasure boats, is highlighted by the fact that only one aftermarket electronic device is generally available on the market, that device having a cost in the range of $400-$500. Recreational vehicles and pleasure boats at the low end of the cost scale are generally not provided with status indicators for water tanks or waste tanks due to the cost involved. In higher end vehicles and pleasure boats, devices supplied are generally of the mechanical, hybrid electronic/mechanical, or conductivity-type probes which have limited lifespans and suffer from erratic behavior.
It would be desirable to be able to supply to the recreational vehicle industry and pleasure boat industry, a level sensing device which is simple to design and manufacture, and easily installed, which contains no moving parts, which is not susceptible to corrosion and salt water environments, and which is capable of providing reliable operation over a long lifespan. Such a device has not been previously available.